Smectite clays are phyllosilicates of the 2:1 layered type. The smectite group includes inter alia the mineral sub-groups montmorillonite, beidellite, nontronite, saponite, hectorite and sauconite. Among the smectite clays to which the present invention is applicable are the naturally occurring Wyoming variety of swelling bentonite and similar clays, and hectorite, which is a swelling magnesium-lithium silicate. For use in the invention, these clays are preferably converted to the sodium form if they are not already in this form. This can be effected by a cation exchange reaction with a soluble sodium compound. These methods are well-known in the art.
Smectite-type clays prepared synthetically can also be used in the slurries of the invention. Representative of such clays are montmorillonite, bentonite, beidellite, hectorite, saponite and stevensite. Representative hydrothermal processes for preparing synthetic smectites are described in U.S. Pat. No. 3,252,757 to Granquist; U.S. Pat. No. 3,586,478 to Neumann; U.S. Pat. No. 3,666,407 to Orlemann; U.S. Pat. No. 3,671,190 to Neumann; U.S. Pat. No. 3,844,978 and U.S. Pat. No. 3,844,979 to Hickson; and U.S. Pat. No. 3,852,405 and U.S. Pat. No. 3,855,147 to Granquist.
Smectite clays are commercially important minerals. Appropriately processed, smectite clays are excellent viscosifiers, binders, film formers, fabric softeners and retention aid additives in paper making. These clays are platey-type materials having a micaceous structure. They are highly colloidal and readily swell in water to form viscous, thixotropic gels which renders the clays useful as viscosity builders in many industries and applications including, for example, in foundry molding compositions, as suspension aids in agricultural sprays, and for thickening cosmetics. The high surface area generated by swelling in water also makes these clays useful in fabric softening and in paper making applications. In the latter environment smectite clays are particularly useful as retention aids. In such use it is often of interest to intermix an aqueous slurry of the smectite clay with a cellulosic suspension, and thereafter drain the cellulosic suspension.
Most smectite clays are sold as fine powders. As with most minerals, however, these powders are difficult and expensive to handle. They also are subject to considerable "dusting" i.e. to evolution of dust during handling, so that they can in some cases cause environmental problems, and even health risks. To minimize these problems and to provide the user of the mineral with a product which is more ready for the user's application, many minerals are sold or shipped by the manufacturer or distributor as high solids aqueous slurries. Such high solids slurries can be easily stored, shipped, transferred, e.g. pumped and metered, with significantly less capital. expenditure and many fewer problems than are associated with dry mineral powders.
In most applications, nevertheless, it is not economical or practical to ship smectite clays as aqueous slurries because of the large quantity of water normally required in such slurries. This problem arises because smectites are indeed good viscosifiers, whereby it has not generally been possible to produce high solids aqueous slurries by the usual methods. In general, only about 8% -10% solids slurries of good quality swelling smectite can be produced in water, and at solids contents greater than about 10%, the viscosities of the slurries can become so high that they cannot readily be pumped by conventional equipment and gelling upon standing becomes a problem.
Thus, there is a need for aqueous slurries containing substantially greater than 10% by weight of smectite clay, which have viscosities low enough to allow pumping. At the same time the high solids slurry must retain the smectite in a form in which it can be readily activated, i.e. to restore the normal properties associated with a swelled bentonite, such as the viscosifying properties, and the ability to effectively function as a retention aid in paper making.
Among the prior art that is pertinent to the present invention are the following:
Van Fisk, U.S. Pat. No. 4,359,339 proposes the addition of water soluble aluminum salt, such as aluminum sulfate, to permit the making of pumpable aqueous slurries of bentonite of up to 26% by weight of clay, primarily for use in making foundry sand compositions used in the making of molds. While apparently suitable for the purpose disclosed, aluminum salts have not been found to be satisfactory for making smectite clay slurries of higher solids content, which are frequently desired for many purposes, especially when shipping is involved. Van Fisk also points out that certain other ions, such as calcium ion have been known to depress the gelling properties of bentonites in aqueous slurries, but that the properties are not readily reversible upon later dilution, thus limiting the value of such use. While aluminum ion appears to be less subject to this objection, to a considerable extent it suffers from the same problem. All multivalent cations tend to strongly bind the clay platelets together, which prevents dispersion and inhibits performance in applications where good dispersion is needed (viscosifiers, fabric softeners, paper making retention aids).
In recently published International Application, WO92/11218, a smectite clay is provided as an elevated solids aqueous slurry, e.g. up to 50% solids, which is composed of the smectite clay slurried or dispersed in water containing an effective concentration of a monovalent salt. When such a salt is present in the clay slurry, the clay does not swell appreciably, is essentially inactive, and the slurry can be shipped and stored without creating a gelling problem. When this slurry is diluted with fresh water at the point of application to make it of the desired solids content for use, the smectite clay is activated and exhibits the desired properties, such as viscosity, retention, etc.
A concept similar to that of WO92/11218 is disclosed in Cluyse et al., U.S. Pat. No. 5,223,098. Bentonite swelling clay is provided to a paper making mill as a fluid concentrate containing more than 15% bentonite. Swelling of the bentonite is. suppressed by an inorganic electrolyte in the concentrate. The bentonite swells upon dilution either before addition of the cellulosic suspension or after addition. While both WO92/11218 and U.S. Pat. No. 5,223,098therefore disclose high solids smectite slurries which meet the needs above discussed, they are nonetheless sharply focused on use of certain inorganic electrolytes, which can introduce undesired ionic species into the smectite slurry. Furthermore, the efficacy of these salts for use as reversible swelling suppressants at higher smectite solids can be undesirably limited.
In accordance with the foregoing, it may be regarded as an object of the present invention, to provide a method for preparing a time-stable low viscosity, high solids aqueous slurry of a swelling smectite clay.
It is a further object of the invention to provide a method as aforementioned, wherein the smectite reversibly is inhibited from swelling in the resultant slurry, whereby upon dilution in or with water at the point of use, the smectite can be easily reactivated and swollen to generate the large surface area needed for maximum performance in many applications.
It is a further object of the invention, to provide a smectite clay slurry of the foregoing character, which remains pumpable and shippable without prohibitive gelling even at elevated solids content--e.g. up to 47% clay solids.
It is a still further object of the present invention to provide an aqueous smectite clay slurry of up to 47% solids content, which is pumpable and shippable, and in which the swelling properties of the smectite can be readily restored during a typical use in paper making.